Step and repeat camera



March 3, 1970 B. D. ABLES Em. 3,49 ,7

STEP AND REPEAT CAMERA Filed Oct. 18, 1967 16 Sheets-Sheet 5 3rd EX XUNIT lsi EX t XTA YTAt== o v Y 1 SI HOME EXR(XTAL?O,YTAL=)' M I FIG.3I

B. D. ABLES ETAL STEP AND REPEAT CAMERA March 3, 1970 16 Sheets-Sheet 4.

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Filed Oct. 18, 1967 V2 mt m #9 h wt wk mow l6 Sheets-Sheet 5 Filed Oct. 18, 1967 L Ii l o o, 0

FIG. IO

O2 4 \w )0 O m 2 w Wm M O m m 9 m m V m F i /w m I76 FIG. I2

- March 3, 1970 a. D. ABLES ETAL STEP AND REPEAT CAMERA 16 Sheets-Sheet 6 Filed Oct. 18, 1967 Q at VOM hum

vkm

mum

March 3, 1970 a. D. ABLES ETA!- STEP AND REPEAT CAMERA.

16 Sheets-Sheet 7 Filed Oct. 18, 1967 BIO March 3, 1970 Filed Oct. 18, 1967 B. D. ABLES ET AL STEP AND REPEAT CAMERA 16 Sheets-Sheet 8 FIG.3O

l2 MlCROlNCHES I TABLE TRAVEL 4 March 3, 1970 Filed Oct. 18, 1967 TURN ON TPE READER SET "ExPoD" TO zERo I202 SET 'XDLD":

I204 "YoLD"= 250.

TYPE "SPECIAL FORMAT,"

I206 COLUMN a Row HDGS.

v I2IO READ e BIT CHARACTER INTO "XNEXT IS THIS END OF FILE READ a BIT CHARACTER INTO "YNExT" CONVERT '-xNExT" TD MILS. a STORE\ m xNEw" CONVERT"YNEXT" TO MIL. a sTpRE IN YNEW' CALCULATE FRINGE COUNT, POSITION a EXPOSE (FIG.,35)

B. D. ABLEs ETAL 3,498,711

STEP AND REPEAT CAMERA 16 Sheets-Sheet 1O 7 DIAGNOSTICS o0 COMP. SELF CHECK LOAD NO OPERATION" IN ALL INTERRUPT S EXCEPT POW R FAI R sET UP TYPEWRITER INT I RRuPTs READ TAPES COMPUTE LOCATION -7 2 a STORE SET "BLANK" E URN ERO I226 TYPE "RUN COMPLETE" FIG. 32 D TRANSFER FORMAT DATA TO'M'JN'; 'XSPAC". 'YSPAC TO FIG.34b

March 3, 1970' B, D, ABLES mI. 3,498,711

SIEP AND REPEAT CAMERA Filed Oct. 18, 1967 16 Sheets-Sheet 11 FROM FIG. 320 892 TYPE FORMAT NUMBER I132 1152 YP QBM. W INCREASE SIZE MBCD x NBCD H22 "WAL" BYI 896 I Y Is THIS EXCEPTION?) TYPE SPACLNGII 1 1 N I I34 xsPco MILS, "Y sPAcING YSPCD MILS C ALCULATE FOR MAT xTAL" BY I CALCULATE FRINGE COUNT POSITION BI CALCULATE FRINGE COUNT POSITION BI 900 EXPOS'E (F|G.35) ExPoE (F|G.35)

TYPE "EXREQ" N "28 EXPOSURES REQUIRED "44 I 902 cALciLATE N .K' E

FIRST EXPOSURE Q YTA TAL Y 904 I a 906 $TRIKE"B LANK" SET "ExP0o"= -2 I I50 907 I, ETuRNTozERoaExPosE) SET "xow": ZERO I,

908 TYPE "READY @1152 FOR EXCEPTIONS" s51" YOLD"= ZERO SET"XNEW ZERO IS"RUN"'BUTTON ON n T sET" TAI zERo SET YTAL l CALCULATE FRINGE COUNT, POSITION a EXPOSE (FIG.35)

SET "YTAII'BI'XTAC'NI T0 I51. EXCEPTION CALCULATE FRINGE COUNT, POSITION a EXPOSE FIG. 35)

CLCULATE FRINGE COUNT POSITION e. EXPO E (FIG.55I

CALCULATE FRINGE COUNT, POSITION BI EXPOSE (H655) CALCULATE FRINGE COUNT POSITION a ExPoE (FIG.34)

TYPE

' "RUN COMPLETE" SET "YTAL"TO 3rd. EXCEPTION CALCULATE COORDI NATES FIG.32b

Filed Oct. 18, 1967 B. D. ABLES Er AL 3,498,711

STEP AND-REPEAT CAMERA 16 Sheets-Sheet 12 HOME HAVE ALL EXTRA INTERRUPTS BEEN IGNORED 2 N SET XMOT= -100% SET YMOT=O CALCULATE XRETARD a F' NEW x INTERRUPT muons LOAD X COMPARATOR WITH XAIM -XRETARD KEEP RECORD OF THIS ONE SET UP X COMPARATOR INTERRUPT TO GO TO X COMP I NEGATIVE RETARD l IOI6 RELOAD X COMPARATOR WITH XAIM BDISX FIG. 39

Marh 3, 1970 B. D. ABLES ETAI- 3,498,7

STEP AND REPEAT CAMERA Filed Oct. 18, 1967 16 Sheets-Sheet 1s RELEASE BRAKE 832 WAIT lsec.

SET UP XZG INTERRUPT SET won-20% SET YMOT=0 m w maes March 3,1970

5. D. ABLES Er AL I STEP AND REPEAT CAMERA Filed 001;. 18. 1967 ST ATS CALCULATE FRINGE COUNT, POSITION 8 EXPOSE CALCULATE XAIM 6 YAI M X COMP. INT. NEGATIV TRAVEL X COMP. INT. NEGATIVE RETARD (FIG. 40)

l6 Sheets-Sheet 14 X COMP. INT. POSITIVE TRAVEL NEGATIVE Y FIG. 37

SET UP X COMPARATOR TO INTERRUPT ON XAIM GET PRESENT X COORDINATE POSITIONING I Y COMP INT. NEGATIVE RETARD MOVE IN POSITIVE Y FIG. 39)

SET UP SLOW X 2 INTERRUPT March 3, 1970 I Filed Oct. 18, 1967 SLOW X(FIG.37)

SLOW X I(FIG.38I

B. D. ABLES ETA!- STEP AND REPEAT CAMERA MOVE IN POSITIVE X MEASURE VELOSITY WAS IT FAST LAST PULSE? S'ET XMOT 100% ls XVEL sLow? MAKE FORCE MORE NEGATIVE SET XMOT 0 HAVE ALL NEW INTERRUPTS BEEN IGNORED? 16 Sheets-Sheet 15 LOAD COMPARATOR WITH XAIM BRAKING DISTANCE REQUIRED BY DRIVE SYSTEM CALCULATE NUMBER OF X COMPARATOR INTERRUPTS TO IGNORE (XIGNR) SET UP COMPARATOR INTERRUPTS TO GO TURN POS.XPILOT LIGHT ON THE X COMPARATOR INTERFIUPTS? HAVE WE WAITED TOO LONG FOR Y Q TO XCIPT 972 MAKE FORCE MORE POSITIVE March 3, 1970 BLE ETAL 3,498,711

STEP AND REPEAT CAMERA Filed 001;. 18, 1967 posmoumg 16 Sheets-Sheet 16 DURING EXPOSURE SET UP X8|Y(+)& (-)TOLERANCES. SET UP "I066 FOR POSITION AVERAGING W Y M m 1 EXPOSURE FINISHED READ x a Y qpL J NT-ZR a ROUTINE STORE In x a Y (AND UPDATE AVERAGES) I076 I082 N Y N Y :oao mi I078 I086 I084 CALCULATE XFORC= CALCULATE XFORC= CALCULATE YFORC= CALCULATE YFoRc= |REsFo (ERROR)N RESFO(ERROR)N+ *IRESFMERROR)" RESF0(ERR0R)N+ xFRlci XFRIC XFRIC] XFRIC l i I SET XMOT=XFORC SET YMOT=YFORC I087 I099 \IS Y wm-uu TOLERANCE? INCREMENT WlTHlN 'NCREMENT TOLERANCE TA ""1. I094 I I00 ERROR TALLY INITIALIZE ERoR TALLIES FIG. 4!

United States Patent O Int. Cl. G03b 27/42 US. Cl. 355-53 1S Claims ABSTRACT OF THE DISCLOSURE A film support table is movable in the X and Y coordinate directions by X and Y drive systems. A laser interferometer and fringe counter detects movement of the table in the X and Y coordinate directions by fringe counts, A projection system simultaneously projects a plurality of images along parallel optical paths onto the film carried by the table after the table is moved to each of a plurality of predetermined exposure positions. The projection system includes a lens stage and a transparency stage which may be positioned at selected heights above the table. The table and the transparency stage include a common banking system for precisely locating plates at predetermined spatial positions relative to the respective optical paths. A reference detector system detects when the table is at a Zero reference position so that the counters can be reset. A digital computer is programmed to compute the coordinate of each exposure position and then, based on the current barometric pressure, compute the distance in fringe counts from the reference position to the first exposure position. The computer then operates the drive system in such a manner as to move the table to the exposure position by continuously computing the position and velocity of the table from the readings of the fringe counters. Then the table is maintained at the exposure position during the exposure period by continuously determining the position of the table from the fringe counters and operating the drive system to produce forces for correcting the positional error.

This invention relates generally to step and repeat cameras, and more particularly, but not by way of limitation, relates to such a camera for producing a set of photolithographic masks for fabricating large arrays of semiconductor devices.

A semiconductor device, such as a transistor, is usually fabricated by a series of diffusion steps. Each diffusion step involves applying a coat of photosensitive polymer, known as photo-resist, over a silicon dioxide layer on the surface of the semiconductor substrate. A photomask is pressed against the surface of the photo-resist and the photo-resist exposed to light. When the photo-resist is photographically developed, selected areas of the photoresist are removed to expose the underlying silicon dioxide. The exposed silicon dioxide is then removed by an etching fluid which does not attack the photo-resist to expose the underlying semiconductor material. The photo- 'resist is then stripped from the silicon dioxide and impurities diffused into the areas of the semiconductor material exposed by the openings in the silicon dioxide layer. A new silicon dioxide layer is either grown over the exposed portion of the semiconductor material during the diffusion process, or is subsequently deposited, and the procedure repeated for the next diffusion step.

Each successive diffusion is typically made either into only a. portion of a previous diffusion, or into a different area of the semiconductor slice so that a different photomask is required for each diffusion step. Each photomask is typically a square of flat glass with a photo- 3,498,711 Patented Mar. 3, 1970 graphically fixed high resolution emulsion on one face which has opaque and transparent areas. Since the face of the photomask carrying the fixed emulsion is pressed directly against the slice, the patterns on the photomask must be actual size, which may involve geometries from as large as tenths of inches to as small as tens of microinches, although line widths on the order of forty microinches are generally considered to be the ultimate limit when using silicon dioxide as the masking layer.

Semiconductor material is more easily grown, handled and processed as disk-shaped slices having a nominal diameter of about 1.5 inches and a thickness of about ten milliinches. For this reason, a large number of semiconductor devices are typically fabricated simultaneously on each slice by the same process steps. It is also common practice to fabricate semiconductors, diodes, resistors, and capacitors for a complete circuit on the same semiconductor substrate, and then interconnect the components by leads patterned from a metal film deposited on the surface of a silicon dioxide layer by the same photolithographic process. Openings are provided in the oxide layer where the metal leads must make contact with the individual active components. The fabrication of integrated circuits usually requires a large number of diffusion steps, and thus a larger number of photomasks for the diffusion steps, and in addition requires an extra photomask to pattern the metal film to form the interconnecting leads. It is also common practice to simultaneously fabricate a large number of integrated circuits on each individual slice of semiconductor material by the same process steps.

It is impractical, if not impossible, to produce a photomask for a large array of either discrete devices or integrated circuits by drawing the entire mask on an enlarged scale and then photographically reducing the entire mask. However, the basic portion of each mask relating to a particular device, group of devices, or an integrated circuit can be originated on a much larger scale, and then optically reduced to a light image of actual size. Then the light image can be stepped over a photographic plate to produce the complete photomask. However, it is vitally important that the light image be precisely located at each successive exposure position with great precision. Otherwise, the successive photomasks will not completely register and the yield will be low.

One method for overcoming this problem involves producing all photomasks of a set simultaneously in a multibarreled step and repeat camera. Then the same positional errors will occur in all masks of the set and the masks will prefectly register. However, this is not practical. Each photomask is good for only a limited number of exposures, for example from twenty to forty. Since a relatively large number of the slices prove defective at an early stageof the process, a much larger number of the photomasks used in the early steps of the fabrication process are required than the number of photomasks used in the latter steps of the mask. Thus, in normal high volume production, the method would result in wasting a. large number of photomasks in a short period of time.

Integrated circuits are widely used as the storage elements and as the logic gates for digital computers and automated control systems. As a result, large numbers of the individually packaged integrated circuits are often interconnected by printed circuits,'or other similar techniques, into a large system. In the last few years, yields have increased to the point where it is practical to fabricate a large number of integrated circuits on a single slice of semiconductor material, test the circuits in situ on the slice, and then interconnect only the good circuits into an array by one or more levels of thin film leads deposited over the slice. However, from one-fourth to onethird of the circuits on a slice may be faulty, and the faulty circuits occur at random positions on the slice. This means that a very large number of different combinations of good circuits can result, A customized photomask, or set of photomasks, must therefore be generated to pattern the thin film lead patterns on each individual slice. This would be highly impractical using conventional techniques. The Wiring masks can be generated by a computer controlled system. But such a system presupposes that each component or circuit is located at a predetermined position on the slice with considerable accuracy. Otherwise, a short or open circuit may be produced at some point where the lead pattern does not register with the circuits, and the entire array would then be faulty.

There is, therefore, a pressing need for a system for generating photomasks in which the position of each pattern on the mask is located with an accuracy on the order of a few microinches. The very best systems heretofore available for positioning the table of a step and repeat camera, or any other movable stage such as those used for automatically positioning machine tools, have positional accuracy on the order of forty microinches, thus requiring an improvement of about an order of magnitude. Further, prior step and repeat cameras are capable of producing masks only about 1.5 inches square, although semiconductor slices about three inches in diameter are now available. On the order of one thousand individual integrated circuits may be placed on a slice having a nominal diameter of about one inch, and on the order of ten thousand circuits can be placed on a slice having a nominal diameter of three inches without decreasing the circuit size. This large number of exposures would take an extremely long period of time using previous step and repeat cameras,

This invention is concerned with an improved step and repeat camera for automatically generating one photomask or a complete set of photomasks required for producing a relatively large array of integrated circuits, or the like, in which the positional accuracy of each successive exposure is on the order of a few microinches. The camera has the capacity to produce a large number of masks simultaneously, specifically eighteen, with any selected reduction power within a wide range, specifically up to X. The camera includes a table for supporting a plurality of photographic plates that are movable in the X and Y coordinate directions of a horizontal plane to be selectively moved to any desired exposure position.

A multiple axis projection system includes a lower stage for supporting a lens system with interchangeable lens barrels, and an upper stage for supporting a set of master transparencies and light sources. The stages may be set at any desired vertical height above the table with great precision by a drive system which separately positions and locks each end of the respective stages, thus eliminating the requirement for a precision drive system.

In a specific embodiment, the stages are supported by a pair of upright support means disposed on opposite sides of the table. Each upright support means includes a threaded rod fixed in a vertical position. Each end of each stage is suported on the respective rod by a sheave threaded on to the rod. A drive system is provided to rotate the sheaves to raise and lower the ends of the stages. Each upright support means also includes a vertically disposed brake rod, which is engaged by a brake carried by the adjacent end of each of the stages, As a result, first one end and then the other end of each stage can be positioned at the proper height and then locked in place by the brake. The precise position may be determined by a calibrated rod disposed vertically adjacent the ends of the stages.

In another specific embodiment of the invention, a single drive motor is carried by each stage and is coupled to simultaneously rotate the two sheaves supporting the respective stage. The normal play in the drive system permits first one end of the stag to be Positioned y .ting the brake at that end, and then he other end positioned separately and the second brake set.

The table and the upper stage also include a common banking system for supporting the master transparencies and the photographic platesin predetermined position relative to the respective optical axis. This coupled with the vertically adjustable stages and the replaceable lens system permit great flexibility of operation.

More specifically, the banking system at each point includes banking means for positioning one surface of each photographic plate in a plane normal to the respective optical axis, and three banking points disposed to engage two adjacent edges of the photographic plate to position the photographic plate in predetermined relationship in the X and Y coordinate directions and in predetermined rotative position relative to the optical axis.

In a more specific embodiment of the invention, the banking systems in the table and in the upper stage each includes means for precisely positioning a multiple plate carrier which may be removed from the table or stage and which includes means for precisely positioning each photographic plate within the carrier.

These and other novel features believed characteristic of this invention are set forth in the appended claims. The invention itself, however, as well as other objects and advantages thereof, may best be understood by reference to the following detailed description of an illustrative embodiment, when read in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a front elevational view of a step and repeat camera constructed in accordance with the present invention;

FIGURE 2 is a plan view of the camera of FIGURE 1 with the upper stages removed to better illustrate the movable table;

FIGURE 3 is a simplified isometric view of the support and drive means for the movable table of the camera of FIGURE 1;

FIGURE 4 is a front elevational view of a portion of the table of the camera of FIGURE 1 partially broken away to reveal details of construction;

FIGURE 5 is a plan view, partially broken away, of the portion of the table shown in FIGURE 4;

FIGURE 6 is an end view, partially broken away, of the portion of the table shown in FIGURE 5;

FIGURE 7 is a sectional view taken substantially on lines 77 of FIGURE '5;

FIGURE 8 is a partial sectional view showing a detail of construction of the portion of the table shown in FIG- URE 5;

FIGURE 9 is a plan view, partially broken \away to show details of construction, of a multiple plate carrier for the camera of FIGURE 1;

FIGURE 10 is a rear end view of the carrier of FIG- URE 9;

FIGURE 11 is a side View of the carrier of FIGURE 9, partially broken away to show details of construction;

FIGURE 12 is a front end view of the carrier of FIG- URE 9, partially broken away to show details of construction;

FIGURE 13 is an enlarged plan view of one corner of the carrier of FIGURE 9;

FIGURE 14 is an enlarged sectional view taken substantially on line 14-14 of FIGURE 9;

FIGURE 15 is an elevational view of the outer face of one of the uprights of the camera of FIGURE 1;

FIGURE 16 is a sectional view taken substantially on lines 1616 of FIGURE 15;

FIGURE 17 is an elevational view of the inner face of the upright shown in FIGURE 15;

FIGURE 18 is a sectional view taken substantially on lines 1818 of FIGURE 17;

FIGURE 19 is a top view of the upright shown in FIG- URE 15;

FIGURE is a sectional view taken substantially on lines 2020 of FIGURE 15;

FIGURE 21 is a top view of the upper stage of the camera of FIGURE 1;

FIGURE 22 is a front elevational view of the upper stage shown in FIGURE 21;

FIGURE 23 is a bottom view of the upper stage shown in FIGURE 21;

FIGURE 24 is an enlarged view of the portion broken away in FIGURE 22;

FIGURE 25 is a sectional view taken substantially on lines 25-25 of FIGURE 21;

FIGURE 26 is a top view of the lower stage of the camera of FIGURE 1;

FIGURE 27 is a sectional view taken substantially on line 2727 of FIGURE 26;

FIGURE 28 is a schematic block diagram of the control system for the camera of FIGURE 1;

FIGURE 29 is a more detailed schematic block diagram of a portion of the control circuit illustrated in FIG- URE 28;

FIGURE 30 is a graph illustrating the operation of the portion of the control circuit shown in FIGURE 29;

FIGURE 31 is a schematic diagram which serves to illustrate the operation of the step and repeat camera of FIGURE 1;

FIGURES 32a and 32b, taken together, are a simplified flow diagram of the program of the computer shown in FIGURE 28 which is used to control the step and repeat camera illustrated in FIGURE 1;

FIGURES 33-41 are flow diagrams illustrating subroutines within the program represented in FIGURES 32a and 32b; and

FIGURE 42 is a graph which plots the force applied for a given positional error in order to maintain the table at a predetermined position during exposure.

Referring now to the drawings, a step and repeat camera constructed in accordance with the present invention is indicated generally by the reference numeral 10 in FIG- URE 1. The camera 10 is mounted on a massive concrete block 12 which is suspended from springs (not illustrated) for isolating the block 12 from the vibrations of the earth. The springs are adjustable so that the concrete block can be leveled. If desired, a pneumatic, self-leveling system can be employed. A base casting 14 is mounted on the concrete block 12 by legs 16. A lower granite block 18 is supported on the base casting 14 by three triangularly spaced threaded rods 20 and nuts 24 which rest on casting 14. The upper surface 22 of block 18 is highly planar and is disposed precisely level by adjustment of the nuts 24 on the threaded rods 20 which rest on the base casting 14. A film support table, indicated generally by the reference numeral 26, is movable in X :and Y coordinate directions over the planar surface 22 of the lower granite block 18.

A pair of uprights 28 and 30 are connected to the base casting 14 and extend upwardly on either side of table 26. Upper and lower stages 32 and 34 are mounted on the uprights 28 and 30 for adjustable movement in the vertical direction. The upper stage 32 supports a pair of light houses 36 and 38 each of which contains nine light sources. Each light source includes a lamp and a lens system to project light along eighteen separate optical axes. The upper stage has facilities for supporting :a master transparency for each optical axis. The lower stage 34 supports a reducing lens for each of the optical axes, and a bellows 40 for each optical axis extends between the upper and lower stages. The table has provision for supporting a photographic plate on each optical axis so that it will be exposed by the image produced by directing light from the source through the respective transparency and reducing lensonto the photographic plate. Each of the transparencies carries the pattern required for a dif ferent photomask used for the different steps of the semiconductor fabrication process. When table 26 is indexed to successive exposure positions, all plates carried by the table are simultaneously exposed so that the exposures will have the same positional errors. All of the. patterns on the photomask can then be made to register simultaneously. The eighteen separate optical axes permit a set of photomasks for an eighteen step fabrication process to be produced.

An important aspect of the present invention is to be able to position each exposure on each film plate at any desired location within a field of travel several inches square, with a positional tolerance of only a few microinches. This not only requires positioning of the table 26 within that tolerance, but also dictates that the apparatus 10 be located in a room where the temperature is maintained constant within a fraction of a degree. Otherwise, expansion of the mechanical parts of the camera will move the transparencies or plates by an amount greater than the specified limits. Similarly, vibrations set up in the machine either from the earth or from within the machine may cause elongations and contractions which would result in the inability to meet the tolerances. These problems are compounded by the very large size of the camera 10 required in order to achieve the large field of travel and a high photographic reduction ratio of as much as 20:1.

The table 26 is comprised of a granite block 42 which supports a metal casting 44. The granite block 42 is supported by four conventional constant pressure air bearings 46 which ride on the surface 22 of granite block 18. Each of the air bearings 46 has a planar bottom surface disposed adjacent to the highly planar surface 22 of the lower granite block 18. Gas, typically nitrogen, is pumped under a constant pressure through the center of each air bearing 46 so that the table 26 is continuously supported by a very thin layer of gas, typically on the order of two microns thick. As a result, the table 26 can be moved over the supporting granite block 18 with a minimum of friction. The gas supply and the individual pressure regulator provided for each air bearing are not illustrated.

Movement of the table 26 is precisely controlled by a guide and drive system which includes a first guide means formed by glass bars 48 and 50 which are mounted on the lower granite block 18. The edge faces 48a and 50a of the bars 48 and 50 are optically flat and are precisely aligned, and the opposite edge faces 48b and 50b are substantially fiat and parallel to the optically fiat faces. Anintermediate stage is formed by granite slabs 52 and 54 which are rigidly interconnected by a third granite slab 56. The slabs 52 and 54 are disposed on opposite sides of the guide rails 48 and 50 and the third slab 56 bridges over bars 48 and 50. Slabs 52 and 54 are supported by pairs of air bearings 53 and 55, respectively, which ride on surface 22 of block 18. A pair of inverted U-shaped yokes 58 and 60 are fixed to the bridge slab 56 and extend downwardly to stand off from the opposite edge faces of guide rails 48 and 50. The yoke 58 carries a fixed air bearing 62, which rides on the optically flat edge face 48a, and a pneumatically biased air bearing 64 which rides on the opposite face 48b and continually biases the fixed air bearing 62 against face 4811 with a constant force. Similarly, the yoke 58 has a fixed air bearing 66 which rides on the optically fiat edge face 50a, and a pneumatically biased air bearing 68 which rides on the opposite face 50b to continually force air bearing 66 against the reference face with a constant force. Thus the intermediate stage is free to move only in the X coordinate direction and is retained at a predetermined Y coordinate over its entire travel Within the design tolerance of a few microinches.

A second guide means is formed by glass bars 70 and 72 mounted on slabs 52 and 54 and have optically flat surfaces 70a and 72a which are aligned precisely at right angles to the optically fiat surfaces 48a and 50a. The opposite faces 70b and 72b are substantially flat and substantially parallel to faces 70a and 72a. A second pair of inverted U-shaped yokes 74 and 76 are fixed to opposite edges of the granite block 42, and have fixed air bearings 78 and 80 which ride on the optically fiat surfaces 70a and 72a, and pneumatically biased air bearings 82 and 84 which ride on edge surfaces 70b and 72b to bias the fixed bearings against the reference surfaces with a constant force.

The granite block 42, and hence the table 26, can be moved in the X direction along guide rails 48 and 50 by means of an X axis drive system comprised of a printed circuit motor 86, which is mounted on upright 28, and drives a wheel 88 which frictionally engages one edge of a drive bar 90. The bar 90 is connected to the intermediate stage by a rod 92. A pair of idler rollers 94 are spring biased against the opposite edge of drive bar 90 to maintain a substantially constant force between the drive wheel 88 and the drive bar 90. The opposite end of the drive shaft of printed circuit motor 86 is provided with a pneumatically operated disk brake which is represented schematically at 96.

The granite block 42, and hence table 26, can be moved in the Y coordinate direction by a second printed circuit motor 98 which drives wheel 100. Wheel 100 frictionally engages one edge of a bar 102 which is connected to yoke 76, and therefore to block 42, by a rod 104. The Y axis drive motor 98 and idler rollers 106 are mounted on slab 54 by a suitable means represented by bracket 108. A pneumatically operated brake 110is also provided on the shaft of the printed circuit motor 98. Thus printed circuit motor 86 moves the table 26 in the X coordinate direction, and printed circuit motor 98 moves the table in the Y cordinate direction. As will hereafter be pointed out in greater detail, the brakes 96 and 110 are used only when the system is not in operation and are not used to position the table during exposure.

The casting 44 of the table 26 is shown in detail in FIGURES 48. The casting 44 is adapted to receive a pair of multiple plate carriers, each indicated generally by the reference numeral 120, in precisely predetermined positions relative to the optical axes. As will hereafter be evident, at least four of the film plate carriers are required for full operation of the camera system. One of the film carriers 120 is illustrated in detail in FIGURES 914. Each film carrier is comprised of a base plate 122. A peripheral side wall 124 is integral with the base plate 122 and extends around the entire periphery of the base plate. A lid 126 is connected to the peripheral side wall 124 by hinges 128 and 130.

The carrier 120 has nine identical compartments formed by interior walls 132, which are also integral with the front plate 122, and the peripheral side wall 124. Aligned square openings 122a and 126a are provided in the base plate 122 and lid plate 126 at each compartment to permit light to be projected through a film plate disposed in the compartment. Each compartment is adapted to receive a standardized square glass photographic plate 134 and to hold the plate in a precisely oriented position relative to the carrier. Orientation longitudinally of the optical axes, which may be considered the Z axis, and also pitch orientation about the X and Y axes, is provided by three studs 136 which project into each compartment from the base plate 122. The film plate 134 is oriented along the X and Y directions, and also in rotation about the Z axis, by a pair of banking lugs 138 and 140 which are pivotally mounted on pins 142 and 144, respectively, and a third lug 146 which is pivotally mounted on a pin 148. The edges of lugs 138, 140 and 146 are straight along the dimension extending longitudinally of the edge of the film plate so as to engage the edge of the plate 134 along a substantial distance, but are rounded in the direction normal to the film plate so as to engage only the center of the edge of the film plate 134. This curvature can best be seen in FIGURE 12.

Two adjacent edges of the film plate 134 are biased against the banking lugs 138, 140, and 146 by an assembly comprised of springs 150 and 152 and an elbow-shaped member 154 which engages the corner of the film plate opposite the edges which abut the banking lugs. The elbow-shaped member 154 is retained in position when the film plate 134 is removed by a pin 156 which is received in an oversized hole (not illustrated)-in the elbow-shaped member 154. The pin 156 has a head larger than the oversized hole to retain the member 154 in place on the pin.

Each film plate 134 is urged downwardly against the three positioning studs 136 by leaf springs 160 which are carried by the lid plate 126 and engage the glass plate 134 directly over each of the studs 136. The lid plate 126 is held against the cumulative force of the leaf springs 160 by a pair of fasteners 161. Each fastener is comprised of a strap 162 which is fixed to the lid plate 126. An aperture 164 in each strap receives the rounded end of a stud 166 which is slidably disposed in the side wall 124 and is biased outwardly by a leaf spring 168.

Each of the film platecarriers 120 has a pair of flat banking surfaces 170' and 172 formed on the outer surface of one peripheral side wall 124, and a third banking surface 174 formed on the adjacent side wall. The banking surfaces 170, 172 and 174 on each carrier 120 are in precisely predetermined relationship to the banking lugs in each compartment of the carrier. The base plate 122 has a number of precision ground reference surfaces 176, shown in dotted outline in FIGURE 9, which lie in a common plane. The ends of all of the banking studs 136 in all of the compartments also lie in a common plane disposed parallel to the plane of the reference surfaces 176. A handle is attached to the peripheral wall 124 to facilitate handling the carrier 120.

The film plate carriers 120 are used to carry both the unexposed film plates for table 26, and also the master transparencies, for the upper stage 32 through which light is projected. The carriers 120 are easily loaded merely by placing the carrier on a table with the reference surface 176 down, releasing the latch assemblies 161, and raising lid plate 126. The film plates 134 may then be placed in the respective compartments merely by manually moving the elbow-shaped member 154 against the force of springs 150 and 152, placing the plates on the studs 136 with the photosensitive emulsion or the phototransparency face down, and releasing the elbow-shaped member 154. The springs 150 will then bias the plate 134 securely against the banking lugs 138, 140 and 146 to precisely position the plate relative to banking surfaces 170, 172 and 174. After all of the plates 134 are loaded in the respective compartments in this manner, the lid plate 126 is closed and the fasteners 161 latched so that the springs 160 securely hold the plates 134 in place against reference studs 136 which precisely orient the plates parallel to the plane of reference surfaces 176. The plate carrier 120 can then be easily taken to the camera 10 and inserted as will presently be described.

Referring once again to FIGURES 4-8, the casting 44 of table 26 is adapted to receive a pair of the plate carriers 120, although only one carrier is illustrated in the drawings. The casting 44 has a W-shaped top plate 200 forming two openings 202a and 202b each of which is slightly larger than the array of nine openings 12211 in the base plate of a carrier 120. The carriers 120 can be inserted into openings in the front of the casting 44 and be slid into position beneath openings 202a and 202b on rails 208 and 210 extending along opposite sides of the openings. A pair of banking lugs 212 and 214 are pivotally mounted on pins 216 and 218 at the right hand side of each of the openings 202a and 202b and are spaced to engage the machined reference surfaces 170 and 172, respectively, on a carrier 120. A third banking lug 220 is pivotally mounted on pin 222 at the rear of each opening 202a and 202b and is spaced to engage the reference surface 174 on the carrier 120. The carrier'120 is biased against the banking lugs 212, 214 and 220 by a lever 224 whichis pivoted on a pin 226, and is biased against the 

